Concrete is a well-known, highly versatile building material comprising cement, aggregate, and water. The mixture is initially prepared in an aqueous slurry that is then allowed to cure in a mold or form. The cement, upon hydration, binds the aggregate in a hardened matrix having considerable compressive strength. Tensile strength may be achieved by embedding steel reinforcement bars in the concrete. Typically, the aggregate includes a gradation of particle sizes to minimize the amount of cement required. The particular size proportions are dictated by geometric considerations and may be specified in construction materials specifications and the like.
There are many building applications where relatively low density concrete is a suitable or useful material. Low density concrete has the advantages of light weight and favorable insulation properties, although it lacks the full strength of normal concrete.
Generally there are two ways to achieve a low density concrete. The first way is to use a low density aggregate such as pumice or other lightweight rock. However, the use of lightweight aggregate material is not always feasible, since it is generally unavailable in many locations where lightweight concrete may be required. The second way of making low density concrete is to add foam to the concrete mix. The foam introduces small air bubbles into the slurry so that when the concrete cures, the air bubbles remain trapped therein, thus providing a so-called "cellular concrete" having a decreased overall density. The use of cellular concrete overcomes the supply problem associated with the use of volcanic aggregates, and further allows an increased degree of control over the density of the finished product. However, cellular concrete typically suffers from prohibitive shrinkage and cracking during the curing operation. Accordingly, cellular concrete has found use in floor fill and roof deck applications, providing insulation and a degree of fire protection, but due to the shrinkage and cracking, it is generally unsuitable for use as a structural material.